When it comes to conserving energy, California is one of the most efficient states. But it’s no longer the best.

That honor goes to Massachusetts for the second year in a row, according to the sixth annual report on the subject from the American Council for an Energy-Efficient Economy.

But the Golden State has been a leader for decades, according to the group, which goes by the less-unwieldy ACEEE. Utilities in the state offer energy efficiency programs for customers and are expected to save nearly 7,000 gigawatt-hours of electricity between 2010 and 2012.

Other top performers include New York, Oregon and Vermont. And by dedicating more funds to conserving energy last year, Oklahoma, Montana and South Carolina enjoyed major efficiency improvements.

States such as Mississippi, North Dakota and West Virginia still need help, according to ACEEE.

“Energy efficiency improvements help businesses, governments, and consumers meet their needs by using less energy, saving them money, driving investment across all sectors of the economy, creating much-needed jobs, and reducing environmental impacts,” said ACEEE Executive Director Steven Nadel in a statement.

Other groups involved in the so-called green economy are out in force Thursday attacking Mitt Romney’s statements about the clean-tech and environmental industry.

“And these businesses – many of them have gone out of business,” Romney said. “I think about half of them … a number of them happened to be owned by people who were contributors to your campaigns.”

The BlueGreen Alliance, a coalition of labor unions and environmental groups, responded by saying that the investments helped create or sustain at least 1 million jobs and leveraged private investment at the rate of $3 of private capital for every government dollar.

“These investments in clean energy, energy efficiency, transportation infrastructure, electrical transmission, and rebuilding American manufacturing have paid important dividends over the last several years,” said Executive Director David Foster in a statement. “They represented a significant down payment on America’s future competitiveness.